Polyimide and a semiconductor prepared therefrom

ABSTRACT

A process is disclosed for making circuit elements by photolithography comprising depositing an antireflective polyimide or polyimide precursor layer on a substrate and heating the substrate at 200° C. to 500° to provide a functional integrated circuit element that includes an antireflective polyimide layer. The antireflective polyimide layer contains a sufficient concentration of at least one chromophore to give rise to an absorbance sufficient to attenuate actinic radiation at 405 or 436 nm. Preferred chromophores include those arising from perylenes, naphthalenes and anthraquinones. The chromophore may reside in a dye which is a component of the polyimide coating mixture or it may reside in a residue which is incorporated into the polyimide itself.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of earlier application Ser. No.08/315,801 filed Sep. 30, 1994, now U.S. Pat. No. 5,441,797, which isitself a division of application Ser. No. 08/054,500, filed Apr. 27,1993, now U.S. Pat. No. 5,397,684.

FIELD OF THE INVENTION

The invention relates to antireflective polyimide layers used inphotolithography to fabricate semiconductor devices.

BACKGROUND OF THE INVENTION

Photolithography using ultraviolet light is a fundamental technology inthe production of semiconductor devices. In the integrated circuit (IC)lithographic process, a photosensitive polymer film is applied to thesilicon wafer, dried, and then exposed with the proper geometricalpatterns through a photomask to ultraviolet (UV) light or otherradiation. After exposure, the wafer is soaked in a solution thatdevelops the images in the photosensitive material. Depending on thetype of polymer used, either exposed or nonexposed areas of film areremoved in the developing process. The majority of Very Large ScaleIntegration (VLSI) exposure tools used in IC production are opticalsystems that use UV light. They are capable of approximately 1 μmresolution ±0.5 μm (3σ) registration, and up to 100 exposures per hour;they are commonly operated at 405 nm (so-called H-line) or 436 nm(so-called G-line).

A frequent problem encountered by resists used to process semiconductordevices, is reflectivity back into the resist of the activatingradiation by the substrate, especially those containing highlyreflecting topographies. Such reflectivity tends to cause standing waveripples and reflective notches, which interfere with the desired patternin the photoresist. The notches are particularly bad when the support ormetallurgy is non-planar.

The problem is illustrated in FIG. 1, which depicts a substrate 1 onwhich has been formed a metal pattern 3. The metal has been covered witha polyimide dielectric 5 and the polyimide layer planarized. A resist 7has been deposited and is being exposed through a mask 9. Incidentradiation passes through the apertures in the mask and, in an idealsituation, exposes only those areas 11 directly in line with theapertures. Unfortunately when the metal pattern 3 is highly reflective,as it usually is, the reflected light from the metal and, to a lesserextent, from the substrate impinges on areas of the resist not intendedto be exposed.

The art discloses two basic approaches to the problem: (1) change thewavelength of the radiation and (2) incorporate some sort of radiationabsorber into or under the photoresist. The first approach is awkwardand expensive because it requires a new tool set. The second approachand its addendant drawbacks are illustrated in FIGS. 2 and 3. In FIG. 2a dye containing a chromophore that absorbs at the appropriatewavelength is incorporated in the resist layer; this cuts down onreflected radiation but also on resist sensitivity. In FIG. 3 a dyecontaining a chromophore of appropriate absorption is incorporated in aspecial layer 13 beneath the resist 7; this adds to the processadditional steps for the deposition and removal of the layer.

A superior process could be envisioned if it were possible toincorporate a dye into the polyimide layer itself (FIG. 4). While thisis a fine idea in theory, in practice it is not straightforward. Sincethe polyimide will remain as part of the semiconductor device, themodified polyimide layer must be deposited as a normal polyimide layerwould be, and then it must survive subsequent curing, planarization andmetal deposition cycles in which the temperature exceeds 400° C. TypicalUV-absorbing dyes such as curcumin and bixin, when incorporated intopolyimide films give rise to dielectric films that are not stable above300° C. Typical pigments that might be thermostable are insoluble andgive rise to problems of homogeneity. Thus there is a need for anantireflective polyimide layer that processes normally and that isextremely thermally stable.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an antireflectivedielectric layer that will survive high temperature processing.

It is a further object to provide a polyimide composition that absorbsradiation in the 400 to 450 nm range and that is stable above 400° C.

It is a further object to provide a photolithography process thatsubstantially eliminates problems of reflectivity without adding anyprocessing steps.

These and other objects, features and advantages are realized in thepresent invention Which relates generally to a process of making asemiconductor structure in which a polyimide is used as an interleveldielectric. According to the process, a chromophore is introduced intothe polyimide film. During subsequent patterning steps, the modifiedpolyimide film absorbs radiation so that it does not reflect and exposethe resist in undesired areas. A suitable chromophore must absorbradiation in the 400-450 nm region. Its host molecule must be thermallystable at temperatures greater than 400° C. and soluble as part of themixture with the polyimide precursor (polyamic acid or polyamic ester)material.

In one aspect, the invention relates to a process for making circuitelements by photolithography comprising:

(a) depositing an antireflective layer on a substrate. Theantireflective polyimide layer comprises a polyimide, polyamic acid,polyamic ester or combination thereof and contains a sufficientconcentration of at least one chromophore to give rise to an absorbancesufficient to attenuate radiation at 405 or 436 nm; and

(b) heating the antireflective layer at 200° to 500° C. to provide afunctional circuit element that includes the antireflective polyimidelayer.

As a practical matter, the process will usually include the intermediateor subsequent steps normally attendant to photolithographic processes,namely depositing a photoresist over the polyimide layer, imaging thephotoresist with actinic radiation, preferably at 405 or 436 nm,developing the photoresist, using it to pattern a layer beneath theresist and removing the resist. Alternately, the photoresist can bedeposited over the uncured antireflective layer and when the layercomprises a polyamic acid, it can be developed along with thephotoresist. The polyimide is then cured after removing the photoresist.

Preferred chromophores, which may occur either in a dye that is acomponent of the polyimide layer or in a recurring unit in a polyimidepolymer, include those chromophores arising from naphthalenes,anthraquinones or perylenes.

In another aspect the invention relates to a thermostable polyimide filmcomprising a polyimide and a thermostable dye. The dye has a molarextinction coefficient (ε) greater than 5000 between 400 and 450 nm anda solubility of at least 2 g/L in an inert, organic solvent. A preferredsolvent is N-methylpyrrolidone (NMP). After cyclization, the filmexhibits less than 1%, and preferably less than 0.5%, decrease in weightwhen heated at 450° for 20 minutes. One group of suitable dyes includesprecursor amic acids which form materials such as perylene red. Thecyclized materials are members of the genus of formula I ##STR1##

The precursors are of formula V ##STR2## wherein R¹ and R² areindependently chosen from the group consisting of phenyl; phenylsubstituted with one or more lower-alkyl, lower-alkoxy, halogen ornitro; naphthyl and naphthyl substituted with one or more lower-alkyl,lower-alkoxy, halogen or nitro and R¹⁰ is hydrogen or alkyl of one tofour carbons.

Other suitable dyes, such as precursors VIII to indanthrene brilliantorange, give rise to members of the genus of formula II ##STR3## whereinAr⁵ and Ar⁶ are independently chosen from the group consisting ofphenyl, anthraquinone, phenyl substituted with lower-alkyl,lower-alkoxy, halogen or nitro; naphthyl; and naphthyl substituted withlower-alkyl, lower-alkoxy, halogen or nitro. Still other dyes aremembers of the genus of formula III ##STR4## and their precursors XII##STR5## wherein Ar⁷ is selected from the group consisting of phenyl;naphthyl; phenyl substituted with one or more lower-alkyl, lower-alkoxy,halogen or nitro; and Ar⁸ is an anthraquinone residue.

The invention also relates to semiconductor devices comprising at leastone functional IC and a polyimide film as described above.

In another aspect the invention relates to a polyimide comprisingrepeating units of formula IV ##STR6## wherein Ar¹ is selected from thegroup consisting of ##STR7## Ar² is selected from the group consistingof ##STR8## Ar³ is selected from the group consisting of residuesencompassed by Ar¹ plus ##STR9## Ar⁴ is selected from the groupconsisting of residues encompassed by Ar² plus ##STR10## m from zero to100; and n is from 1 to 100;

with the proviso that at least one of Ar³ and Ar⁴ must be other than asubstituent chosen from the groups Ar¹ add Ar².

Preferred polyimides include those in which m is 90 to 99 and n is 1 to10, those in which Ar³ is ##STR11## and Ar⁴ is Ar², and those in whichAr³ is Ar¹ and Ar⁴ is ##STR12##

In another aspect, the invention relates to blended polyimidescomprising a first component, which is a conventional polyimidecomprised of the groups Ar¹ and Ar², and a second component which is apolymide of formula IV.

In another aspect the invention relates to semiconductor devicescomprising at least one functional IC and a polyimide film as describedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 are schematic cross-sections of devices of the art; and

FIG. 4 is a schematic cross-section of a device according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Four criteria must be met in providing an antireflective polyimide layerthat will remain a part of a functional IC device: (1) the layer mustfunction normally as a dielectric, (2) the layer must behave like anormal polyimide in fabrication processes, (3) the layer must be stableabove 400° C., and the layer must have a sufficient absorbance toprovide a practical barrier to reflected radiation. In accordance withthe invention, there are provided two approaches to such a layer: (1)incorporate a dye meeting the foregoing criteria into a conventionalpolyimide precursor solution or (2) incorporate a chromophore into thepolyimide polymer itself without impairing the mechanical and electricalproperties of the cured polymer.

Dyes that have been found suitable for incorporation into the precursormix include soluble amic acid precursors to perylene red ##STR13## andindanthrene brilliant orange (formula II; Ar⁵ =Ar⁶ =phenyl). The dyesare not usually incorporated into the spin-coating mix in their imide(cyclized) forms, but rather, like the polyimide itself, areincorporated in the formulation as their amic acid or ester precursorsand are then cyclized in situ when the polyamic acid or ester is cured.In addition, condensation products of perylenetetracarboxylic acid witharomatic amines and condensation products of anthraquinone diamines witharyl orthodicarboxylates have been found to possess suitablechromophores while exhibiting compatible solubility in normal castingsolvents such as N-methylpyrrolidone (NMP). Moreover the films formedfrom these dyes in combination with conventional polyamic acids andesters are surprisingly often more thermally stable than thecorresponding unmodified polyimide layers. The absorbance through a onemicron layer will normally be at least 0.05 and preferably greater than0.3 at 405 or 436 nm. This is sufficient to usefully attenuate mostradiation that would give rise to a reflection problem.

The dyes may be made by reacting perylene dianhydride (PDA) with twomoles of the appropriate monoamine to form the amic acid V, which, whenheated, cyclizes to the imide, as shown in Scheme A: ##STR14##

A mixture of amines can be used to make perylene-imides having differingR groups.

Indanthrene-type (fused benzimidazole) dyes of formula II may besynthesized analogously from a cyclic dianhydride and two moles of anorthodiamine VII, as shown in scheme B: ##STR15##

As before, a mixture of amines will produce compounds of formula IIhaving differing Ar groups. Two examples are fused imidazoles IX and X,made from PMDA and diaminonapthalene or diaminoanthraquinone: ##STR16##

The dyes of genus III may be prepared similarly from two moles of acyclic monoanhydride and one mole of a non-ortho diamine: (Orthodiaminestend to cyclize to fused benzimidazoles as above.) ##STR17##

In all of the foregoing Schemes, the initial reaction is carried out toproduce the amic acid (e.g. V, VIII, XII) which is coated on thesubstrate in solution and then cured to the final product afterdeposition. The important features of all three classes of dyes arethat, probably as a result of analogous structures, their precursorshave compatible solubility to that of polyamic acids and esters for spincoating the resins, and the chromophores can be modulated by appropriatesubstitution to achieve the necessary absorbance between 400 and 450 nm.

From consideration of Schemes A, B and C, particularly B, one canenvision that by using a UV-absorbing dianhydride or diamine one couldmake polyamic acids, similar to VIII, which could then be inserted intoconventional polyamic acid precursors for polyimide films: ##STR18##

This may be accomplished by mixing the anhydrides and diamides in theproper stoichiometry for the desired absorbance and physical properties.We have found that about 1 to 10% of one component having theUV-absorbing chromophore produces useful polyamic acids for theproduction of polyimide films of 0.5 to 5 μm thickness. If thinner filmsare desired, the proportion of absorbing precursor can be increased. Theactual amount to be used will depend on the extinction coefficient ofthe particular chromophore and the amount of absorbance needed for aparticular application.

An example of a film according to Scheme D wherein the stoichiometryincludes no component XVI is shown below: ##STR19##

Similar polyamic acids can be made using a UV-absorbing diamine, such as1,5-diaminoanthraquinone as the XVI component in place of or in additionto XVa. However, adjustments in stoichiometry of the components must bemade to account for the low reactivities of aromatic diamines with XVa.

Alternatively, one can make UV-absorbing polyamic acids and makephysical mixtures with conventional polyamic acids or esters.Conventional polyamic acids and their constituent diamine/dianhydridecomponents are described, for example, in U.S. Pat. No. 4,480,009(column 18 to 22) the disclosure of which is incorporated herein byreference. These mixtures can then be cured to make blended polyimidesrather than copolymers, i.e. physical mixtures rather than covalentcompounds.

The polyamic esters may be prepared by methods well-known in the art,for example by reacting the anhydride with an alcohol followed bythionyl chloride and then the diamine. The esters have certainadvantages when it is desired to heat the polyamic ester precursor (forexample to drive off solvent) without having it cyclize to a polyimide.The ester can then be cyclized to the polyimide by raising thetemperature.

EXAMPLE 1 (Formula II: Ar⁵ =Ar⁶ =naphthalene)

To a solution of 15.82 g (100 mmol) of 2,3-diamino naphthalene in 100 mLof NMP was added 3.4 g (50 mmol) of 1,4,5,8-naphthalenetetracarboxylicdianhydride (NDA), and the formulation was mixed on a roller mill for 16hours to form the 2:1 amic adduct. Ten grams of this solution was addedto 75 g of a 16% solution of polyamic ethyl ester (PAETE). The NMPsolution was spin coated on a quartz substrate to a thickness of 2 μmand heated at 350° C. for 20 min. The resulting film exhibitedabsorbance of 0.68 at 405 nm and 0.35 at 436 nm; thermogravimetricanalysis (TGA) indicated a weight loss of 0.18% when heated from 350° to450° over a period of 100 minutes.

EXAMPLE 2 (Formula II: Ar⁵ =Ar⁶ =anthraquinone)

The procedure of example 1 was followed using 1,2-diaminoanthraquinonein place of 2,3-diaminonaphthalene. The resulting film exhibitedabsorbance of 0.90 at 405 nm and 0.45 at 436 nm; thermograviometricanalysis showed a weight loss of 0.12% on heating from 350° to 450° over100 minutes.

EXAMPLE 3 (Formula IV: Ar¹ =phenyl, Ar² =xylyl, A³ =perylene, Ar⁴=xylyl, m=99 and n=1)

3,4,9,10-Perylenetetracarboxylic dianhydride (0.625 g) (1.59 mmol) (PDA)was reacted with 10 g (73.42 mmol) of m-xylylenediamine by combiningreagents and mixing the reactants on a roller mill. After 18 hours 150mL of NMP and 15.667 g (71.93 mmol) of pyromellitic dianhydride (PMDA)were added to the mixture and the reactants mixed again for 24 hours.This copolymer produced good amic acid films that imidized to a redpolyimide according to the procedure described in example 1. Thepolyimide film exhibited absorbance of 0.11 at 405 nm and 0.13 at 436nm; TGA showed a weight loss of 0.15% under the usual program.

EXAMPLES 4-7 (Formula IV: Ar³ =phenyl, Ar⁴ =anthraquinone, m=0) plusPAETE

As examples of copolymer blends (physical mixtures of polyimides), fourdiaminoanthraquinones were individually reacted with one equivalent ofPMDA at 0.5 mmol/mL in NMP to produce amic acid solutions of 18.5%solids. The solutions were mixed 1:9 with PAETE, spin coated and heatedat 350° C. to give copolymer blends suitable for device coating.

    ______________________________________                                                 Amino                                                                Example  Substitution    post 350° C. λ.sub.max                 ______________________________________                                        4        1,2             406                                                  5        1,4             400,480                                              6        2,6             350,490                                              7        1,5             432                                                  ______________________________________                                    

1,5-Diaminoanthraquinone (Example 7) gave rise to films with absorptionmaxima at 432; these are particularly advantageous for mercury G-linephotolithography.

EXAMPLE 8 (Formula IV: Ar¹ =phenyl, Ar² =Ar⁴ =xylyl, Ar³ =perylene,m=98, n=2)

A copolymer was prepared by adding excess m-diaminoxylene toperylenedianhydride. The resulting 2:1 amine/anhydride adduct formed asa dark precipitate which was dissolved and incorporated as a block alonga polyamic acid copolymer chain with PMDA as the major anhydride link. Astoichiometry of 2.1% perylene gave the resulting copolymer significantred color in the imidized film. This copolymer provides good filmquality, but the UV absorption maxima are at 496 and 533 with only ashoulder at 464.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that other changes in form and details may bemade wherein without departing from the spirit and scope of theinvention.

We claim:
 1. A polyimide comprising repeating units of formula ##STR20##wherein Ar¹ is selected from Group A consisting of ##STR21## Ar² isselected from Group B consisting of ##STR22## Ar³ is selected fromGroups A and C consisting of ##STR23## Ar⁴ is selected from Groups B andD consisting of ##STR24## wherein m is from zero to 100; n is from 1 to100; and with the proviso that at least one of Ar³ and Ar⁴ must be fromGroup C or D.
 2. The polyimide of claim 1 wherein m is 90 to 99 and n is1 to
 10. 3. The polyimide of claim 1 wherein Ar³ is ##STR25## and Ar⁴ isAr².
 4. The polyimide of claim 1 wherein Ar⁴ is ##STR26## and Ar³ isAr¹.
 5. The polyimide of claim 4 wherein Ar⁴ is ##STR27##
 6. Asemiconductor device comprising:(a) a semiconductor substrate having atleast one functional integrated circuit; and (b) a polyimide filmaccording to claim 1.